FIG. 1 shows the working principles of a vibrating yaw rate sensor with respect to a moving object 100 and the Coriolis Force, which is represented by the following formula:F=2Ω×VHere, Ω is the angular velocity vector of the rotating coordinate system XYZ, and V is the velocity vector of the moving object 100 within the rotating coordinate system XYZ. In this regard, as shown in FIG. 1, the moving object 100 will appear to an observer, who is fixed with respect to the rotating coordinate system XYZ to accelerate in a direction that is perpendicular to the velocity vector V and the angular velocity vector Ω.
In a MicroElectrical Mechanical System (MEMS) gyroscope which operates according to the above principles, the Coriolis force may change the stress on micromechanical beam contained therein. The change in stress may be measured, for example, using principles of piezoelectronics or piezioresistance.
MEMS gyroscopes may be used in high-volume applications, including such applications as automotive and consumer electronics. For example, MEMS gyroscopes may be used in electronic stability programs, rollover sensors, navigation systems, computer gaming input devices, camcorder stabilization, electronic toys, etc. In this regard, the use of integrated circuit (IC) fabrication technologies and MEMS batch processes may provide low-cost, super compact and highly integrated gyroscopes that are more cost-effective and convenient than traditional mechanical or optical gyroscopes in large-scale and cost-sensitive applications.
MEMS gyroscopes may operate in the manner of a vibrating gyroscope. For example, Putty and Najafi, “Tech. Dig. Solid-State Sensor and Actuator Workshop”, Hilton Head Island, S.C., June 1994, pages 213 to 220, refer to a vibrating ring gyroscope; Bernstein et al., “MEMS '93”, Fort Lauderdale, Fla., February 1993, pages 143 to 148, refer to tuning fork gyroscope; and Juneau et al., “Tech. Dig. 9th International Conference on Solid State Sensors and Actuators (Transducers '97)”, Chicago, Ill., June 1997, pages 883 to 886, refer to a torsion-vibrating gyroscope. To reduce the size of torsion-vibrating MEMS gyroscopes, it would be desirable to use a minimum number of resonant structures to sense the yaw rate in more than one direction. However, because it has been difficult to couple vibration mode in two orthogonal directions, there are no MEMS gyroscopes which use a single tuning fork resonator structure to sense more than one axis yaw rate, except the torsion-vibrating disc-based gyroscope referred to in the Juneau et al reference.
To sense the yaw rate in more than one axial direction, two individual vibrating gyroscopes may be placed on the same chip, each gyroscope being arranged to sense the yaw rate in one of two orthogonal directions. In this instance, because of the inherent variation in the manufacture process, even two identically manufactured vibrating gyroscopes will not have exactly the same resonance frequency. Therefore, such a system requires two separate detecting and control circuitry.
FIG. 8 shows a one axis system 800, which includes MEMS portion 810 and an evaluation electronics 820. The MEMS portion 810 includes a drive MEMS 811 and a coriolis MEMS 812. The evaluation electronics 820 includes drive electronics 821, coriolis detection electronics 822, and a demodulation control circuit 823.
The drive electronics 821 drives the MEMS portion 810 to resonate at its resonance frequency with a defined amplitude in order to make use of the Coriolis effect, which is proportional to the velocity of the moving mass and results in a force that will be measured by the coriolis detection electronics 822. In this instance, the drive electronics 821 is provided with automatic gain control (AGC). As indicated by the dashed line connecting the drive MEMS 811 and the drive electronics 821, the circuitry may be optionally operated in closed loop.
The coriolis detection electronics 822 measures the force that is generated because of the Coriolis effect. Since the drive MEMS 811 is oscillating at the drive resonance frequency, the coriolis signal is an AC signal at exactly the drive resonance frequency. If the coriolis resonance frequency is exactly the same as the drive resonance frequency and the quality factor Q of the coriolis MEMS is high, the coriolis signal should have a high signal-to-noise ratio. The coriolis detection electronics 822 includes a front end 824, a demodulator 825 and an LP 826. The coriolis detection electronics 822 may optionally be operated in closed loop, as indicated by the dashed line connecting the coriolis detection electronics 822 to the coriolis MEMS 812.
The demodulation control circuit 823 provides a control signal to demodulate the output of the coriolis front end 824 with a signal at the drive resonant frequency and the proper phase information to filter out the corolis signal in the coriolis loop. In this instance, a phase lock loop (PLL) may be used to generate the control signals for the demodulation. However, such a phase lock loop may consume a considerable amount of chip area and power. Although use of the phase lock loop (PLL) is not mandatory, the phase lock loop (PPL) may replace the extra clock unit.
FIG. 9 shows a two-axis system 900, which duplicates the electronics of the one-axis system 800 of FIG. 8. With such duplication, the two-axis system 900 requires two separate drive resonance frequencies, which are not guaranteed to be equal.
FIG. 10 shows an exemplary dual-axis yaw rate system that has only one drive electronics and only one control circuitry demodulation.
FIG. 11 shows an exemplary dual-axis yaw rate system which uses only one front end followed by a multiplexer for the coriolis loop.